In recent years, because of growing awareness about global environmental problems, respective automobile manufacturers have reduced the weight of a vehicle body with the aim of improving the fuel economy. Also, from a viewpoint of safety of passengers, safety standard against collision of an automobile has become stricter, and durability of a member to a shock also has been required. Therefore, in recent automobiles, the use ratio of high strength steel sheets has further increased, and particularly in vehicle body skeletal members and reinforce members that require corrosion resistance, hot-dip galvanized steel sheets or alloyed hot-dip galvanized steel sheets having high strength have been positively applied. Under expansion of use applications of high strength steel sheets, the required properties have risen, and improvement of workability of a base metal has been required further more in hard-to-form members.
As a steel sheet having both of strength and workability, there is a dual-phase steel sheet (may be hereinafter referred to as a DP steel sheet) mainly composed of ferrite having high elongation and martensite exerting high strength. Also, as a high strength steel sheet achieving both of high workability and high yield ratio, in the Patent Literature 1 for example, a hot-dip galvanized high-tensile steel sheet is disclosed that has the strength of 780 MPa or more, excellent elongation, and the yield ratio of 60-80% which is achieved by making the average grain size of ferrite 5.0 μm or less and making the average grain size of the hard second phase 5.0 μm or less. According to the technology disclosed in the literature, precipitation strengthening elements of Ti and Nb are added to strengthen precipitation and to strengthen miniaturization of the structure, however Ti and Nb are required to be added by a great amount, and therefore there is a problem from the viewpoint of the cost.
In the meantime, with respect to a high strength hot-dip galvanized steel sheet for a vehicle body skeleton, energy absorption performance in collision is required in addition to workability, and a technology for manufacturing a steel sheet with high yield strength or high yield ratio at a low cost has been required. However, the DP steel sheet exhibits a low yield ratio, and does not achieve both of high yield ratio and high workability. Also, in the Patent Literature 1, a steel sheet achieving both of high yield ratio and high workability is shown, however there is a problem on the manufacturing cost. Therefore, materialization of a technology that allows manufacture of a high strength galvanized steel sheet exhibiting high yield ratio and excellent workability at a low cost is desired.